This invention relates to the automatic optical testing of transparent bodies, in particular articles such as bottles or flasks of glass or plastics materials, at high operating rates of the order of 15,000 articles per hour and more.
Many different types of optical testing are known in the industry. Certain very precise checks can only be carried out at relatively low operating rates because it is necessary to use light emitters having a narrow beam and to grip the articles one by one to turn them in front of the apparatus. In many situations, however, such a careful inspection is not necessary and much higher operating rates can be achieved. In such circumstances, it is preferable to observe the articles while moving, without contact, and to limit the checks to those for the most harmful defects, which are those most easily seen. Such defects include deformations, major defects such as fractures or heavy glazing resulting from thermal shocks or handling that are liable to cause breakages in the production line, opacity attributable to the presence of relatively large stones or bubbles, "chicken-costs" that are dangerous to the user, etc.
Such visual tests are presently carried out by direct observation in transparency on a clear background. The defects observed produce more or less luminous stains which are inscribed inside a more uniform image, the darkened outline of which indicates the profile of the object, or which even alter this profile in the case of a broken or deformed article.
The articles to be examined are carried aligned in a row, spaced apart by a suitable spacer means and transported by a horizontal table conveyor through an examination apparatus. The examination apparatus comprises a light source which illustratively is a simple screen providing a luminous background on one side of the row of articles and on the other side of the row a fixed optical receiver system comprising an electronic camera having a short response time. A lens forms a real image of the article on a photosensitive surface of the camera which is raster scanned by electronic means to convert this image into a succession of quantified electrical signals. Analysis of these signals permits detection of anomalies in the articles and dimensional measurements of the outline of the article. In this way it is possible to characterize the nature and magnitude of the defects and, where appropriate, to discard the article.
Since the edges of the image appear darker and perturbed, defects can be detected only in a clear central zone of about 40.degree. to 50.degree. on either side of the optical axis of the examination apparatus and not in the marginal zones. As a result, it is generally necessary to carry out multiple examinations of the articles along two axes intersecting at 90.degree. or even along three axes intersecting at 60.degree.. If the inspections are three in number, at least one rotation of the article is necessary between two of the inspections because it is not possible to align the examination apparatus in a direction too close to the axis of the conveyor.
In view of the variation in shape of the articles to be tested, each camera requires a minimum on the order of 40,000 pixels, of equal or at least very similar sensitivities, to obtain an overall image of satisfactory resolution. Such a camera is expensive both in capital cost and in maintenance since failures lead to a relatively high frequency of replacement of the photosensitive matrices.
Accordingly, it is preferred to use a linear camera in which the photosensitive area is a simple strip or chain of photosensitive diodes, arranged in the form of a single vertical column comprising, for example, 2.sup.8 =256 diodes or 2.sup.9 =512 diodes. In this case, the complete inspection of the article is caused by passing it on the horizontal conveyor in uniform straight translation in front of the apparatus so that successive vertical slices of this article form their image on the vertical column of diodes one after another. The diodes are read by a rapid vertical electronic scan; and the spacing of the analysis columns produced by the successive readings is a function of the speed of passage of the articles and the rate of the scan.
One notable disadvantage of this arrangement is that the dimensional measurements cannot be read until the end of one complete scan of the image. While this scan time is quite short, an article moves enough during the scan for the accuracy of the measurement to suffer from perturbations of mechanical origin such as vibrations, variations in speed of the conveyor, slippage, and of optical or even electrical origin such as shadows, variations in luminous intensity and so on.